Syntax

You can define a constant by using the
define()-function or by using the
const keyword outside a class definition as
of PHP 5.3.0. While define() allows a constant to be
defined to an arbitrary expression, the const keyword has
restrictions as outlined in the next paragraph.
Once a constant is defined, it can never be
changed or undefined.

When using the const keyword,
only scalar data (boolean, integer,
float and string) can be contained
in constants prior to PHP 5.6. From PHP 5.6 onwards, it is possible to
define a constant as a scalar expression, and it is also possible
to define an array constant. It is possible to define
constants as a resource, but it should be avoided, as it can
cause unexpected results.

You can get the value of a constant by simply specifying its name.
Unlike with variables, you should not prepend
a constant with a $.
You can also use the function constant() to
read a constant's value if you wish to obtain the constant's name
dynamically.
Use get_defined_constants() to get a list of
all defined constants.

Note:
Constants and (global) variables are in a different namespace.
This implies that for example TRUE and
$TRUE are generally different.

If you use an undefined constant, PHP assumes that you mean
the name of the constant itself, just as if you called it as
a string (CONSTANT vs "CONSTANT"). An error of level
E_NOTICE will be issued
when this happens. See also the manual entry on why
$foo[bar] is
wrong (unless you first define()bar as a constant). This does not apply to (fully) qualified constants,
which will raise a fatal error if undefined. If you simply want to check if a
constant is set, use the defined() function.

These are the differences between constants and variables:

Constants do not have a dollar sign ($)
before them;

Prior to PHP 5.3, Constants may only be defined using the
define() function, not by simple assignment;

Constants may be defined and accessed anywhere without regard
to variable scoping rules;

Constants may not be redefined or undefined once they have been
set; and

Constants may only evaluate to scalar values. As of PHP 5.6 it is possible
to define array constant using const keywords and as of
PHP 7 array constants can also be defined using define()
You may use arrays in constant scalar expressions
(for example, const FOO = array(1,2,3)[0];),
but the end result must be a value of allowed type.

As opposed to defining constants using define(),
constants defined using the const keyword must be
declared at the top-level scope because they are defined at compile-time.
This means that they cannot be declared inside functions, loops,
if statements or try/
catch blocks.

Note:

Constants defined using the const keyword are always
case-sensitive, while constants defined using define()
may be case-insensitive.